It is generally necessary in the operation of vehicle drives because of relatively narrow usable speed range of internal combustion engines, transmission and gear devices which allow load-dependent or speed-dependent operation of the drive train are used with such engines. Since, on account of their performance characteristics, internal combustion engines can in principle only provide a low torque at low rotational speeds, it is also necessary to provide a separating device or clutch which makes possible the starting of a vehicle or running of an engine. Furthermore, in particular in the case of vehicles, synchronizing devices which allow shifting between different transmission stages during operation or during the journey are conventionally used in the gearbox. Corresponding gearboxes are of a complex structural design.
Mechanical, manually operated gearboxes without a clutch or synchronizing device are also used in vehicles with internal combustion engines, or in combination with hydrostatic gearboxes, where the hydraulic pump and hydraulic motor of which are connected in a closed circuit. The hydrostatic gearbox serves in this case merely as a clutch or torque converter. When synchronizing devices are absent, it is often necessary to stop the vehicle for changing the transmission ratio, or at least reducing the speed to a minimum in order to avoid considerable mechanical wearing of the manually operated gearbox.
It is therefore the object of the present invention to provide a drive train in which the hydrostatic gearbox acts together with an electronic control unit to make rapid and dependable separation or synchronized gear changing possible in a simple way without further coupling or synchronizing devices in the gearbox. At the same time, the object of the method of controlling the drive train is to control the change between different transmission ratios of the gearbox during the journey by means of digital electronics.